In a conventional lithographic printing process, an inking system is used to feed ink to the image areas of the printing plate and a separate dampening system is used to dampen the non-image areas of the printing plate. The water provided for dampening is more or less uniform across the press, while the ink input is regulated according to the image coverage of each printing zone and hence varies across the press. Such conventional processes have numerous drawbacks. The print quality is highly sensitive to the quality of the dampening systems, which are complex, expensive, difficult to maintain and take up valuable space. Great skill is required of the press operators to ensure that the proper ink/water ratio (i.e., ink/water balance) is maintained across the press during printing.
A relatively long start-up time is required before the ink/water balance reaches a steady state, and print quality varies during the start-up time. The time for the press to reach a steady state after a change in the ink feed rate is inversely proportional to the image coverage of each printing zone. Press operators commonly adjust the ink feed rate before the press has reached a steady state condition and hence end up chasing after a target print density constantly throughout an entire press run. This also accounts for inconsistent print quality. When the optical print density is lower than the target value, it could be caused by either insufficient ink supply or too much water supply. It requires a skilled crew to make the correct adjustment. Failure to do so may eventually result in tremendous print waste.
Ink input requirements vary across the press, which adds complexity to the printing process control, especially for a large newspaper press which may have as many as a thousand ink keys that need to be adjusted.
The aforementioned difficulties associated with presses having separate ink supply and dampening systems have prompted the development of systems using a single fluid for both inking and dampening: emulsion ink. Emulsion inks used in lithography are made from an emulsion of an oil-based ink and a water-based fountain solution. The emulsion ink is applied to a printing plate (typically mounted on a plate cylinder) having distinct image areas and non-image areas. The image areas have an oleophilic material, such as an oleophilic polymer, disposed on the surface thereof, so that the oil-based ink will adhere thereto for subsequent transfer to a printing substrate, such as a paper web. The non-image areas have a hydrophilic material, such as an aluminum oxide, disposed on the surface thereof, so that the water-based fountain solution will adhere thereto, thereby forming a protective film over the non-image areas, to prevent ink from adhering thereto. A principal advantage of the use of emulsion inks is that emulsion inks can eliminate the need for a separate system to dampen the printing plate and hence eliminates printing problems associated with keeping the ink/water properly in balance. Also, using emulsion inks simplifies the printing process by eliminating the need for many ink keys that would otherwise be required in presses using separate dampening and inking systems, i.e., to account for variations in image density.
However, a major drawback of the use of emulsion inks is that emulsion inks are often unstable (i.e. the oil-based ink and water-based fountain solution separate into distinct liquid layers). Such instability is undesirable because it interferes with ink transfer. For example, if the emulsion ink is not stable enough, the oil-based ink and water-based fountain solution will separate prematurely, before reaching the printing plate, resulting in scumming and wash marks, as water released from the emulsion ink will interfere with ink transfer by (a) reducing the amount of emulsion ink fed to the printing plate and (b) flushing across image areas of the printing plate. However, if the emulsion ink is overly stable, it will not release a sufficient amount of water to the printing plate to keep the non-image areas of the printing plate free of ink. Accordingly, the emulsion ink is formulated to have a stability that is within a "window" between being too stable and too unstable for satisfactory lithographic printing. It has been found that suitable emulsion inks have a water content of at least 35% by weight.
Also, because the viscosity of lithographic inks is relatively high, about 10 to a few hundred poises, lithographic inks generally do not flow freely. As water is dispersed into a matrix of lithographic ink to produce emulsion inks, the flow properties further deteriorate, making the formation of a suitably stable emulsion ink difficult.
Accordingly, when using emulsion ink, it is often necessary to adjust the ink/water balance during operation of the printing press. With existing press configurations, the adjustment will not take effect until the emulsion ink needing adjustment is substantially used up.